To optimize the process, key components must be analyzed, including:
- Control efficiency requirements.
- Oxidizer type.
- System design criteria.
- Potential for self-sustaining operation.
- Secondary energy recovery options.
- Oxidizer efficiency.
A closer look at each of these components will help you maximize oxidizer performance.
Control Efficiency RequirementsFor an emission control system, overall control efficiency is the product of two major items: capture efficiency and destruction efficiency. Capture efficiency includes collection of VOCs at the dryer or oven and capture of fugitive emissions from the entire process system. Destruction efficiency is the level of removal provided once the VOCs are introduced into the emission control device. Today's regulations demand high overall VOC control efficiencies. For many coating applications, overall emissions control requirements of 95% are standard. In certain geographic areas, environmental regulations require overall VOC control efficiencies of 98%.
High VOC capture efficiency is mandatory for most systems and may require enclosures around coating equipment. These coating rooms, known as permanent total enclosures (PTEs), contain or capture 100% of the emitted process solvents, allowing them to be ducted directly to the pollution control system. The exhaust from these enclosures also is sometimes used as makeup air to the oven system. How the PTE exhaust is handled is a function of process conditions and oven design.
Once VOCs have been captured, they are sent to an emission control device. Oxidation systems are end-of-pipe pollution control solutions for emissions from many solvent-based processes. Title V of the Clean Air Act Amendment of 1990 directs states to develop site-permit programs. As implementation of Title V proceeds, more facilities are demanding greater than 99% destruction efficiency on new equipment. Carbon monoxide and nitrogen oxide emissions also are becoming an issue. The desired efficiency rate can vary depending on process requirements. To be certain the most effective results are achieved, users should choose the oxidizer type best suited to a specific application.
Oxidizer TypesOxidation systems are supplied in many designs and configurations, and various types of oxidizers are used to control coating line VOC emissions. The design variations exist to address differences in process conditions, operating schedules, installation space requirements and operating costs. Oxidizers are defined based upon the type of heat exchanger used and the presence or absence of a catalyst within the oxidizer airflow.
Direct Thermal Oxidizers. These systems do not have heat exchangers and can be costly to operate. Most often, they are used only on intermittent-flow applications such as small pilot operations.
Recuperative Heat Recovery Systems. Typically, these systems use a metal shell-and-tube or plate-type heat exchanger to recover the energy used in the oxidation process. The recovered heat is used to preheat process exhaust as it enters the oxidation system.
Recuperative heat exchangers recover 40 to 80% of the oxidation process energy; most system designs fall into the range of 60 to 70% recovery. The successful use of metal recuperative heat exchangers is impacted by several factors, including process exhaust temperatures; system operating temperature requirements; temperature stratification within the unit, which relates to flow turndown; type and concentration of the VOCs treated; and the process operating cycle. These factors affect the unit's efficiency and life.
In some applications, the temperature limitations of the metals in heat exchangers, along with stresses induced by changing process conditions, can reduce the life of recuperative units.
Regenerative-Type Heat Recovery Systems. Regenerative heat recovery systems typically use ceramic media to collect and store energy. The ceramic media is contained in multiple towers or canisters that are interconnected by ducting and a valve system, which directs the incoming exhaust stream among the various ceramic canisters. By switching from one ceramic tower to another (that is, cycling), one ceramic bed can release its energy while the ceramic bed in another tower absorbs energy. Like the recuperative system, regenerative oxidizers use the recovered energy to preheat process exhaust as it enters the oxidizer.
While most regenerative units operate in an energy recovery range of 85 to 95%, they are capable of recovering up to 97% of the energy used in the oxidation process. The ceramic media used in these systems typically is capable of continuous operating temperatures of 1,800 to 1,900oF (980 to 1,040oC). High temperature capabilities and the use of hot-gas bypass systems allow regenerative systems to operate effectively over a range of airflows with VOC concentrations from nearly 0 to 25% of the lower flammability limit (LFL).
Catalytic Oxidizers. These devices use a catalyst to lower the operating temperature required to destroy VOCs. Typical catalyst formulations are precious or base metal beads or monoliths. The catalyst is deposited on a substrate that is placed in the solvent-laden airstream. While catalysts lower operating temperatures and save energy, they are susceptible to poisons and masking agents that can reduce catalyst activity and VOC-destruction capabilities.
System Design CriteriaOnce the capture system has been designed, the next consideration when defining the oxidation system is to quantify the concentration of VOCs and other contaminants in the exhaust stream. Using the design parameters for the oven system, the types of VOCs used in the process can be identified; the minimum and maximum solvent concentrations defined; and the exhaust volumes and temperatures determined. Any particulate that may be generated by the coating equipment or process chemistry also must be identified. These are critical parameters used in the selection of the oxidation system.
The process operating schedule is another important consideration. The variability in process uptime can impact tremendously on the operating efficiency and life expectancy of pollution control equipment. Catalytic oxidizers are well-suited for processes that generate high solvent concentrations and are frequently on- and off-line. Lower operating temperatures can reduce the extreme stress on equipment components. Run-ning schedules also impact oxidation system selection. Regenerative oxidizers are more efficient for low solvent concentrations, large airflows and continuous operation.
The potential for contaminants also must be evaluated. Nonvolatile oxidation products such as silicones, phosphates, clay, glass fibers, resin fragments and other inorganic substances sometimes can enter the exhaust stream. Even in concentrations as low as a few parts per million, these impurities can clog an oxidizer in just a few months, and concentrations as low as 0.1 ppm can mask catalysts in less than one year.
Nothing is more important to an oxidizer's efficient operation than the exhaust rate. As the exhaust rate for a given process condition is reduced, solvent concentration increases. Lower exhaust rates with higher solvent levels reduce the need for fuel in every oxidizer type. Once the LFL has reached six to 12%, however, fuel consumption is minimal for oxidizers with heat exchangers, making further exhaust rate reductions unnecessary.
Self-Sustaining OperationThe self-sustaining condition is a desirable operating condition for any oxidation system. Only through careful optimization of process air requirements can this condition be achieved.
The energy recovered in the oxidation process comes from the energy generated while burning of process solvents plus the energy introduced through an auxiliary burner. Increased energy from the process solvents reduces the auxiliary fuel required to support the oxidation process. Regenerative thermal, regenerative catalytic and recuperative catalytic systems can be designed to operate using only the energy available from the process solvents -- an operating condition called self-sustaining.
In some oxidizer applications, the self-sustaining condition is achieved easily, and energy from the process solvents exceeds the needs of the oxidizer. In these cases, the excess energy could be used to reduce energy demands from the process or other areas in the plant.
Secondary Energy Recovery OptionsSecondary and even tertiary energy recovery systems are becoming more popular as energy costs continue to rise. Energy recovery through the use of air-to-air heat exchangers or thermal fluid systems such as hot oil or water glycol exchangers is a good option. Some facilities have hot/chilled water or steam demands that can be satisfied using a waste heat boiler or adsorption chiller fired by the oxidation system exhaust. In northern climates, facilities can use this ex-cess energy as makeup air for the comfort heating system. Figure 1 shows a generic system that includes secondary and tertiary heat exchangers, where the primary heat exchanger is an integral part of the oxidizer.
When looking at the potential for secondary and tertiary heat recovery systems, potential users should first confirm that process and pollution control system energy demands are met. Only when they are satisfied is it appropriate to investigate alternative energy recovery projects.
Optimal process design only can be achieved by understanding multiple equipment variables and the interrelationships between process heating and oxidation systems. Clearly, oven or dryer design affects emission control design and operation. Optimal solutions balance the competing needs of all variables and keep the total process in mind.